Method of degassing and fracturing coal seams



United States 4 Claims. Cl. 299-16) ABSTRACT OF THE DISCLOSURE Thefracturing and degassing of coal seams is carried out by pressing avolatile liquid consisting of at least one non-combustible halogenatedhydrocarbon having at atmospheric pressure a boiling point of between lC. and +50 0., into a mine gas-containing rock zone including a coalseam, at an elevated pressure sufficient to fracture said coal seam, soas to fracture said zone, the latter being at an elevated temperatureabove the boiling point of the volatile liquid at atmospheric pressure;and releasing said elevated pressure so that said volatile liquid incontact with said fractured zone at said elevated temperature will bevolatilized, thereby forming a gaseous mixture of volatilized liquid andmine gas, and withdrawing the thus formed gaseous mixture from the thusfractured zone, thereby relieving the latter of mine gas.

The present invention relates to a method of degassing and fracturingcoal seams.

Hydraulic fracturing, sometimes referred to as the frac method isutilized for hydraulically breaking or fracturing oil or gas-holdingrock formations in order to improve by such hydraulic fracturing, in oneor more stages, the conditions of fluid flow in the oil or gas holdingformation. For this purpose, viscous liquids, so called fracturingliquids, are used, for instance, jellified crude oils, water or acidswhich are pressed into the deposit at high pressure, the so-calledformation breakdown pressure. The cracks formed thereby in the thusfractured rock are maintained opened by supporting materials, such asspecial sand, aluminum spheres, granulated walnut shells or glass beadswhich are introduced together with the fracturing liquid.

Due to mechanization of underground coal mining, particularly hard coalmining, the rate of advance per unit of time in working up coal seamshas been greatly increased and probably will be further increased in thefuture. With a more rapidly progressing working face, and particularlylong-wall faces, the release of methane gas, or mine gas, from the coalseam and the surrounding rock increases constantly due to release ofrock pressure and crack formations connected therewith. For this reason,in order to maintain adequate safety standards, the operation ofmechanized coal mines must be interrupted from time to time since it isnot possible to eliminate by ventilation the large quantities of minegas, or to maintain the concentration thereof below the permissiblemaximum. Such interruption of the mining operation, quite obviously, isundesirable for technical and economical reasons.

In view of these problems, the suggestion has been developed to degasthe as yet unmined rock and coal seam por tions by hydraulic fracturingand thereby to remove a major portion of the mine gas before therespective coal atent UL! \ilUll "ice seam portion comes wi hin therange of the actual mining operation.

However, the problems involved in the hydraulic fracturing of coal seamsare different from those encountered in the hydraulic fracturing of oilor gas bearing rock, inasmuch as due to the low pressure of mine gas,the fracturing liquid, after having accomplished fracturing of the rockand/or coal seam, in contrast to conditions prevailing in the fracturingof oil or gas bearing rock, will not be automatically, eruptivelyremoved. Thus, it would become necessary to pump out fracturing liquidafter the same has accomplished the fracturing of the coal seam orsurrounding rock. Such pumping over very considerable vertical distancesis an expensive procedure and sometimes technically impossible,particularly if the direction of fracturing planes is downwardlyfollowed the pitch of the fractured layers.

It is therefore an object of the present invention to provide a methodfor the fracturing and degassing of coal searris and surrounding rockwhich can be carried out in an effective, simple and economical manner.

It is a further object of the present invention to provide a method forthe fracturing and degassing of coal seams and surrounding rock whichwill not require the pumping out of the spent fracturing liquid.

Other objects and advantages of the present invention will becomeapparent from a further reading of the description and of the appendedclaims.

With the above and other objects in view, the present inventioncontemplates a method of fracturing and degassing coal seams, whichcomprises the steps of pressing a volatile liquid into a minegas-containing coal seam under a pressure sufficient to fracture suchcoal seam, so as to fracture the coal scam, the latter being at anelevated temperature above the boiling point of the volatile liquid atatmospheric pressure, releasing the pressure so that the volatile liquidin contact with the fractured coal seam at the elevated temperature willbe volatilized, thereby forming a gaseous mixture of volatilized liquidand mine gas, and withdrawing the gaseous mixture from the thusfractured coal seam, thereby relieving the latter of mine gas. 1

Thus, according to the present invention, the liquids which are used forfracturing the coal seam and surrounding rock will be such that the samecan be applied during pressing or pumping into the coal seam area inliquid condition and will be liquid while causing the fracturing of thecoal seam and rock, which, however, after completion of fracturing ofthe coal seam or rock will be volatilized due to the higher temperatureof the coal seam and rock and due to the release of the pressure underwhich the volatile liquid had been introduced. The thusformed gas willmix with the mine gas and the thusformed gaseous mixture can bewithdrawn, for instance sucked out or pumped out through the bore holethrough which the volatile liquid had been introduced. The removal ofsuch gaseous mixtures does not cause the difliculties experienced inpumping out of a fracturing liquid.

It is thus essential according to the present invention that fracturingof the coal seams and the like is carried out by means of low boilingliquids.

Suitable liquids are primarily those which are known as refrigerants,such as halogenated hydrocarbons, for instancemonofiuorodichlo'romethane, monofiuorotrichloromethane, as Well asliquified ammonia carbon dioxide, sulfur dioxide, liquified petroleumgas, etc. Prefer- IIVUIVI ably such liquids will be utilized which arenoncombustible, because they can be manipulated without danger and willreduce the risk of explosions in the mine area. It is also possible toutilize a mixture of several liquids instead of a single fracturingliquid.

Generally, all liquids may be used as fracturing liquids which have atatmospheric pressure a boiling point between about 100 C. and +50 C,Preferred are liquids having boiling points of between C. and +30 C. andbeing noncombustible.

Economic considerations are of great importance in. choosing a suitablerefrigerant or fracturing liquid, in view of the fact that forindividual application frequently several hundred cubic meters of suchliquids are utilized.

The following list of suitable fracturing liquids is given asillustrative only without limiting the invention to the specific liquidsmentioned therein:

Ammonia, carbon dioxide, sulfur dioxide, methylchloride, methylbromide,methylenechloride, monofiuorotrichloromethane, difiuoromethane,trifiuoromonochloromethane, tetrafiuoromethane, ethane and fluorine-freeethane derivatives such as ethyl chloride, ethyl bromide,fluorine-containing ethane derivatives such as trifluorodichloro-ethane,tetrafluorodich'loro-ethane and its symmetric and asymmetric isomers,pentafluoromonochloroethane, trifluoromonochloro-ethane,difluoromonochloroethane, trifiuoro ethane, difluoro ethane, propane andderivatives thereof such as fluorochloro derivatives of propane, forinstance hexafluorodichloropropane, butane and butane derivatives suchas isobutane, fluorochloro derivatives of butane, perfluoro butane,ethylene, halogenated ethylene derivatives, vinyl chloride, vinylbromide, dichloro ethylene, trichloro ethylene, difluoro ethylene,difluoromonochloro ethylene, propylene, cyclic hydrocarbons and theirhalides, for instance cyclopropane and octafiuorocyclobutane, etherssuch as dimethyl ether or diethyl ether, aliphatic amines such asmethylamine, dimethylamine, ethylamine, methylformiate; nitrous oxide,thionylfiuoride, sulfurylfiuoride, sulfur hexafluoride, borontrichloride, certain mixed refrigerants such as mixtures with variableevaporation temperatures and azeotropic mixtures, butylene,methylfiuoride, pentane, carbon disulfide, trimethylamine andtrifiuoro-acetone.

Generally, the method of the present invention is carried out by firstdrilling a bore down to the underground area which is to be fractured,introducing a pipe through the bore and cementing the pipe. The pipe isthen perforated at the depth or level at which fracturing is to becarried out, or it is opened at this level by a sand blast. Thereafter,the refrigerant, preferably combined with supporting material such asaluminum spheres or special fracturing sand is pressed by means of highpressure pumps into the bore hole so as to enter the coal seam orsurrounding rock at the level at which fracturing should be carried out.The pressure must be at least equal to the formation breakdown pressureat this point. After introduction under pressure of the volatile liquidhas been completed (up to several hundred cubic meters thereof may haveto be introduced) the overpressure at the bore hole is released, so thatthe volatile liquid which is in contact with the coal seam orsurrounding rock of higher temperature and which has now beenvolatilized can escape together with the mine gas present in thefractured area, or can be removed therefrom by pumping or suckmg.

The following example is given as illustrative only without limiting theinvention to the specific details thereof.

Example After lowering a bore into the unfractured rock containing thecoal seam, for instance, a 6 inch tube is cemented into the lowerportion thereof. In the area of the coal seam which is to be fractured,for instance 1000 meters below ground, the tubes are opened within oneplane by means of a sand blast. It is assumed that the coal seam has athickness of 1.5 meters and a permeability of 1 md., that the pressureat the deposit will be of the magnitude of a few atmospheres and thetemperature at about 35 C. The petrostatic pressure at this depth willbe about 220 atmospheres and consequently it can be expected that crackformation will be in horizontal direction. In order to obtain asufficient withdrawal area for methane, a fractured area of about 300meters diameter is to be produced which, by using German standardfracturing sand of between 8 and 28 mesh should have a permeability of150 D. The treatment is now carried out by pressing 150 cubic meters ofthe low boiling liquid into the previously slitted coal seam. Frac sandis added to the low boiling liquid in increasing concentration of fromabout to about 300 grams per liter. At an injection rate of between 3and 4 cubic meters per minute, the maximum pressure at the head of thebore will be about 200 atmospheres. Thereafter an additional 12 cubicmeters of the low boiling liquid, however free of sand, is pumped intothe coal seam. After completion of pressing of the volatile liquid intothe coal seam, the overpressure at the bore is released, and theintroduced volatile liquid which now under the influence of the somewhatelevated temperature of the coal seam and surrounding rock will be ingaseous condition, will either escape through the bore under its ownpressure or may be sucked out of the bore with the help of conventionalventilation or pumping arrangements. The gas removed at first,corresponding to up to about 50 percent of the amount of volatile liquidwhich had been pumped into the bore, will consist practicallyexclusively of the volatilized liquid, and thereafter, i.e. after about50% have been removed, will include increasing proportions of mine gas.The concentration and type of the supporting material such as aluminumspheres or glass spheres or frac sand will have to be determined in amanner well known to those skilled in the art, depending on the specificconditions in each given case. The amount of volatile liquid which is tobe used depends on the desired dimensions of the fractured area whichagain will have to be determined based on the specific conditions ineach case.

The fracturing liquid, or at least a considerable portion thereof isthen recovered from the gaseous mixture in conventional manner.

The fracturing liquid which is used in this example is methyl chloride.

The abbreviations mdf and D. in this example mean millidarcy and darcy,respectively. Darcy is the unit of permeability.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended Within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A method of fracturing and degassing coal seams, comprising the stepsof pressing a volatile liquid consisting of at least one non-combustiblehalogenated hydrocarbon having at atmospheric pressure a boiling pointof between C. and +50 C., into a mine gas-containing rock zone includinga coal seam, at an elevated pressure sufficient to fracture said coalseam, so as to fracture said zone, the latter being at an elevatedtemperature above the boiling point of the volatile liquid atatmospheric pressure; and releasing said elevated pressure so that saidvolatile liquid in contact with said fractured zone at said elevatedtemperature will be volatilized, thereby forming a gaseous mixture ofvolatilized liquid and 5 mine gas; and withdrawing said gaseous mixturefrom the thus fractured zone, thereby relieving the latter of mine gas.

2. A method of fracturing and degassing coal seams as defined in claim1, wherein said gaseous mixture is withdrawn from said fractured coalseam by pumping,

3. A method of fracturing and degassing coal seams as defined in claim1, wherein at least a portion of said volatile liquid is recovered fromsaid withdrawn gaseous mixture,

4. A method of fracturing and degassing coal seams as defined in claim1, wherein said. volatile liquid has a boiling point of between about-10 C. and 30 C References Cited UNITED STATES PATENTS OTHER REFERENCESDegasifying Before Mining, Coal Age, August 1961, pp. 75-78 incL, byWilliam M. Merritts,

ERNEST R, PURSER, Primary Examinera

